Corrected an error message

In method_ldexp an error message was "can't convert %s into Float" when it
should have been "can't convert %s into Integer".
Changed test_type_error

This test was not entirely correct:
It expected the raised error of a call such as
"function_taking_two_floats('Bang!')"
to be a TypeError since the first argument was a string. The truth is that (at
least in the math module of ruby 1.9.3) the raised error should be an
ArgumentError because the function is called with one instead of two arguments.
In other words: Checking for an ArgumentError should be done before checking
for a TypeError.
Added test for ldexp

The ruby ldexp function should be able to take a float as it's second argument.
It's just the undecorated python function that should only accept an int as
second argument because it assumes that it was already casted from float to
int.
The changes made to test_ldexp ensure that Math.ldexp can also take a float as
second argument and that it handles it right.