Make httpclient_adapter threadsafe

httpclient_adapter uses some standard Ruby hashes, which produce unexpected results when working in a true multi-threaded (e.g. JRuby) environment. Multiple threads doing puts and deletes on these hashes result in nils sometimes being returned for a properly stubbed request. These errors would manifest as:
webmock-3.9.2/lib/webmock/http_lib_adapters/httpclient_adapter.rb:113:in `build_httpclient_response': undefined method `body' for nil:NilClass (NoMethodError)

Unfortunately these errors are extremly difficult to replicate.

This PR is a follow-on from bblimke#908, and represents the last thread-safety issue we've been able to find.

lib/webmock/http_lib_adapters/httpclient_adapter.rb

@@ -43,6 +43,9 @@ def self.disable!
   end
 
   module WebMockHTTPClients
+
+    REQUEST_RESPONSE_LOCK = Mutex.new
+
     def do_get_block(req, proxy, conn, &block)
       do_get(req, proxy, conn, false, &block)
     end
@@ -57,7 +60,7 @@ def do_get(req, proxy, conn, stream = false, &block)
       WebMock::RequestRegistry.instance.requested_signatures.put(request_signature)
 
       if webmock_responses[request_signature]
-        webmock_response = webmock_responses.delete(request_signature)
+        webmock_response = synchronize_request_response { webmock_responses.delete(request_signature) }
         response = build_httpclient_response(webmock_response, stream, req.header, &block)
         @request_filter.each do |filter|
           filter.filter_response(req, response)
@@ -68,7 +71,7 @@ def do_get(req, proxy, conn, stream = false, &block)
         res
       elsif WebMock.net_connect_allowed?(request_signature.uri)
         # in case there is a nil entry in the hash...
-        webmock_responses.delete(request_signature)
+        synchronize_request_response { webmock_responses.delete(request_signature) }
 
         res = if stream
           do_get_stream_without_webmock(req, proxy, conn, &block)
@@ -100,7 +103,7 @@ def do_get(req, proxy, conn, stream = false, &block)
     def do_request_async(method, uri, query, body, extheader)
       req = create_request(method, uri, query, body, extheader)
       request_signature = build_request_signature(req)
-      webmock_request_signatures << request_signature
+      synchronize_request_response { webmock_request_signatures << request_signature }
 
       if webmock_responses[request_signature] || WebMock.net_connect_allowed?(request_signature.uri)
         super
@@ -184,7 +187,9 @@ def build_request_signature(req, reuse_existing = false)
 
     def webmock_responses
       @webmock_responses ||= Hash.new do |hash, request_signature|
-        hash[request_signature] = WebMock::StubRegistry.instance.response_for_request(request_signature)
+        synchronize_request_response do
+          hash[request_signature] = WebMock::StubRegistry.instance.response_for_request(request_signature)
+        end
       end
     end
 
@@ -193,8 +198,10 @@ def webmock_request_signatures
     end
 
     def previous_signature_for(signature)
-      return nil unless index = webmock_request_signatures.index(signature)
-      webmock_request_signatures.delete_at(index)
+      synchronize_request_response do
+        return nil unless index = webmock_request_signatures.index(signature)
+        webmock_request_signatures.delete_at(index)
+      end
     end
 
     private
@@ -209,6 +216,16 @@ def headers_from_session(uri)
         hdrs
       end
     end
+
+    def synchronize_request_response
+      if REQUEST_RESPONSE_LOCK.owned?
+        yield
+      else
+        REQUEST_RESPONSE_LOCK.synchronize do
+          yield
+        end
+      end
+    end
   end
 
   class WebMockHTTPClient < HTTPClient