fix passing sourcemap iterator in Lazy Referencing

In the body of [`checkLazyReferencing`](https://github.com/apiaryio/snowcrash/blob/master/src/BlueprintParser.h#L304) there is a passing of an iterator which might be uninitialized(in case of not exporting the source map). this mistake also happened in the next functions for traversing blueprint and source map AST, there are few possible solution:

- passing the iterator pointer as the input of the destination function.
- initialize all the iterator, whether exporting source map or not.

this PR suggests passing the iterators pointer themselves as a solution to this problem. as the other way would require unnecessary initialization.


also there are changes in `test-ResourceParser.cc` and `BlueprintParser.h` in order to increase the readability of the code.
> What about the *requestSourceMapIterator here? 

No problem here. This is correct, guarded by the condition, constructing `ParseResultRef`