#252 merging reverse mapping with forward mapping.

There is also an error in the `Mapper#reverse` method class.  The check on constant / expression has been fixed.

Also, the order of forward / reverse method is not relevant (as thought before). This is been assured by adding some tests.
> That's an interesting case as there are two matching "forward" mapping methods. So which one is going to be used as configuration source? Is it determined in any way? You mentioned some "ordering" above, but I'm not sure I'm following.

What the intention is: 
1. if a property-mapping is defined for a mapping method, there' s no issue. MapStruct takes that property mapping
2. if a property-mapping is defined in its reverse counterpart mapping, MapStruct takes the reverse property mapping and switches target-source
3. if a match can be made on names, it takes the name match. 

I agree: order should not matter. And I don't believe it does. Its a time ago, but I think I checked. Otherwise I can add a simple reproducer, by using exactly the same mappings in another mapper, specified in reverse order.

> Btw. I'm a bit unhappy about the reverse mapping feature in general

With that, you mean also the current implementation? I agree. I think we should  make it explicit. Suggestions? Perhaps we can learn from the current qualifier solution. There was a discussion with @agudian on having an annotation on mapping method level. Perhaps such an annotation could also be used to identify that the reverse mapping should be used. So something like: `@MappingMethod( useReverseMappingsAsTemplate=true, qualifier... )`

About this PR. What I'm trying to achieve is that you don't have to redefine mappings you provided earlier, once you decide to do one thing differently in the reverse mapping. 

So, in my case I've got a source bean which has more properties than the target bean, simply because its generated  from standard entities, like a measurement that has certain properties. So lets say, my source bean has property sets: Smap, and Sconstant. Only Smap maps to Target. If Smap is a set of around 20 properties that do not match with the target names, I've got to define 20 mappings.

I want to re-use these same mappings in the reverse mapping and put Sconstant to constants. So, logically, in the reverse mapping I only want to specify the constants and take Smap from its 'forward mapping'.