Ensure compatibility with Nokogiri

Nokogiri v1.12 introduced a breaking change related to namespace
inheritance of reparented child nodes. This commit using the feature added in
v1.12.4 to opt

- for new versions of Nokogiri that support it, set `Document#namespace_inheritance`
- intermediate versions of Nokogiri will be avoided via gemspec version specification
- old versions of Nokogiri will continue to work

See sparklemotion/nokogiri#2320 for more details.

Fixes ebeigarts#30

lib/signer.rb

@@ -65,6 +65,7 @@ def initialize(document, noblanks: true, wss: true, canonicalize_algorithm: :c14
     self.document = Nokogiri::XML(document.to_s) do |config|
       config.noblanks if noblanks
     end
+    self.document.namespace_inheritance = true if self.document.respond_to?(:namespace_inheritance)
     self.digest_algorithm = :sha1
     self.wss = wss
     self.canonicalize_algorithm = canonicalize_algorithm

signer.gemspec

@@ -21,5 +21,5 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency 'rake'
   gem.add_development_dependency 'rspec'
 
-  gem.add_runtime_dependency 'nokogiri', '>= 1.5.1'
+  gem.add_runtime_dependency 'nokogiri', '>= 1.5.1', '!=1.12.3', '!=1.12.2', '!=1.12.1', '!=1.12.0'
 end