Fix bug in dep resolution when requesting a "const T*"

[marcobergamin]

Hello, I have fixed a bug in dependency resolution when requesting a "const T*" after injecting a "T*" as dependency.

Before this fix, if a T* was injected as dependency, requesting a const T* will result in a invalid nullptr. The dependency resolution of objects passed by reference is not affected by this problem.

I have added four tests:

• dependencies_with_reference
• dependencies_with_const_reference
• dependencies_with_pointer
• dependencies_with_const_pointer (this test fails without the fix)

I am new to TMP and this is my first contribute in general so I hope I didn't do anything wrong.