[integrals/meijerint] Fix a bug in _split_mul.

This fixes issue 3761.

_split_mul tries to write an integrand as c*x**a*f, where a and c are
independent of x. However, it did not actually test that a is independent
of x ...

Fix this, and add the test from the issue.
[integrals/meijerint] replace e.has(x) by x in e.free_symbols
Merge pull request #2015 from ness01/3761

Fix issue 3761.