Changed the `diophantine()` function so that it now respects the assumptions
imposed on the symbols. Originally the issue was raised in a so question found
in

http://stackoverflow.com/questions/23301941/how-can-i-declare-natural-symbols-with-sympy

and reported as SymPy issue #7444 by Aaron.

A similar approach as in `solve` is carried out to check for the assumptions.
Each symbol's assumptions are tested against the value returned by the
`diop_solve()` for that variable using `check_assumptions()`.
Remove an unnecessary blank line
Replace `Integer=True` with `integer=True`
Remove unnecessary lines in test case
Add a new test case
Merge pull request #7448 from thilinarmtb/Filter_diophantine_solutions_using_assumptions_issue_#7444

Filter diophantine solutions using assumptions issue #7444