Fix composer module checkmode and change detection

Hi,

While writing and testing playbooks using the composer module, I ran into two issues:
- check mode didn't work, even if it seems supported
- the changed status detection test was not valid
I propose this simple pull request to address those issues.

Regarding my fix proposal for check mode, I think it is suboptimal, because to me the root cause of the bug resides in the **for** loop which treats each module parameter. It might be a cleaner and safer way to build the options for composer by accessing explicitly each module parameters, as the *gem* module does. Nevertheless, I thought this quicker proposal might be better suited for reviewing purpose.