enhance GetterMatcher to match boolean getter 'isSomething'

[argan]

No description provided.

[garcia-jj]

The prefix is is only valid for primitive boolean according Java Language Specification. So the 4th sentence is not valid, because Boolean wrapper should use get prefix.

boolean isXpto() is valid.
Boolean getXpto() is valid.
Boolean isXpto() is not valid.

If you want to make micro optimization you can use element.getParameterTypes().length == 0 sentence as the 1st because is more faster than String.startsWith.

[garcia-jj]

@jonasabreu What you think about this pull request? I have a code that depends on this.

Thank you.