Retain any literal "HTTP-" in header names (#950)

If a literal "HTTP-" exists in a request header name, it will appear as
"HTTP_" in the Rack environment. This shouldn't be removed when
extracting it from the environment.

lib/raven/integrations/rack.rb

@@ -113,7 +113,7 @@ def format_headers_for_sentry(env_hash)
 
           next unless key.start_with?('HTTP_') || %w(CONTENT_TYPE CONTENT_LENGTH).include?(key)
           # Rack stores headers as HTTP_WHAT_EVER, we need What-Ever
-          key = key.gsub("HTTP_", "")
+          key = key.sub(/^HTTP_/, "")
           key = key.split('_').map(&:capitalize).join('-')
           memo[key] = value
         rescue StandardError => e

spec/raven/integrations/rack_spec.rb

@@ -103,6 +103,14 @@
     expect(interface.headers["Version"]).to eq("HTTP/2.0")
   end
 
+  it 'retains any literal "HTTP-" in the actual header name' do
+    interface = Raven::HttpInterface.new
+    new_env = env.merge("HTTP_HTTP_CUSTOM_HTTP_HEADER" => "test")
+    interface.from_rack(new_env)
+
+    expect(interface.headers).to include("Http-Custom-Http-Header" => "test")
+  end
+
   it 'does not fail if an object in the env cannot be cast to string' do
     obj = Class.new do
       def to_s