[Fix #801] Add `context_dependent` style to BracesAroundHashParameters

Before this change, the no_braces style allowed braces if all
parameters in a method call were hash literals. Looking at the
second last parameter makes more sense. Now we go for more strict
checking, never leaving room for options within one style.
Merge pull request #1439 from jonas054/801_braces_for_last_params

[Fix #801] Add a `depending` style to BracesAroundHashParameters
[Fix #1441] Compare AST for whole buffer in AutocorrectUnlessChangingAST

We were already re-parsing the whole buffer to check for syntax
errors. If we check for AST equality instead, we don't need to
check the syntax.