Header, cookie, and form parameters should be optional

According to the spec
(https://github.com/wordnik/swagger-spec/blob/master/versions/2.0.md#parameterObject),
only "path" parameters are required by default, while for
all other parameter types the default value is "false".
Header, cookie, and form parameters should be optional

Unittests fixed

Conflicts:
	modules/swagger-core/src/test/scala/parameter/ParameterSerializationTest.scala
Merge pull request #725 from martoe/params_optional

Fixed the default value for the "required" flag of header-, cookie-, and form-parameters