Implemented a responseIsXml function, to check if the response received is in the format of xml.
Merge pull request #572 from FnTm/1.6

Implemented a responseIsXml function
doneSeeElement issue fix #587
Merge branch '1.6' of github.com:Codeception/Codeception into 1.6
fix to webdebug module
fix to unit test errors
fix to conditional asserts