Fixed a bug where the inflector would replace camelCase strings and...

Fixed a bug where the inflector would replace camelCase strings and disregarding specified acronyms, fixes #8015

activesupport/lib/active_support/inflector/methods.rb

@@ -91,7 +91,7 @@ def underscore(camel_cased_word)
       word = camel_cased_word.to_s.dup
       word.gsub!('::', '/')
       word.gsub!(/(?:([A-Za-z\d])|^)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1}#{$1 && '_'}#{$2.downcase}" }
-      word.gsub!(/([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
+      word.gsub!(/(?!#{inflections.acronym_regex})\b([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
       word.gsub!(/([a-z\d])([A-Z])/,'\1_\2')
       word.tr!("-", "_")
       word.downcase!

activesupport/test/inflector_test.rb

@@ -167,11 +167,13 @@ def test_acronyms_camelize_lower
   def test_underscore_acronym_sequence
     ActiveSupport::Inflector.inflections do |inflect|
       inflect.acronym("API")
+      inflect.acronym("APIs")
       inflect.acronym("JSON")
       inflect.acronym("HTML")
     end
 
     assert_equal("json_html_api", ActiveSupport::Inflector.underscore("JSONHTMLAPI"))
+    assert_equal("namespaced/apis", ActiveSupport::Inflector.underscore("Namespaced::APIs"))
   end
 
   def test_underscore