Use PHPUnit_Framework_TestFailure::exceptionToString if invocation expectations fail

[fabian]

Primarily to display a diff of the mismatching parameters.

If a larger array argument doesn't match the expectation, additional debugging is needed to see passed value.

With this change you get a nice diff for the mismatching argument:

Expectation failed for method name is equal to <string:right> when invoked 1 time(s).
Parameter 0 for invocation SomeClass::right(Array (...)) does not match expected value.
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
 Array (
-    0 => 'first'
-    1 => 'second'
+    0 => 'second'
 )

[edorian]

I was wondering why it didn't nicely diff my failures recently.

Thanks bunch for making that happen also providing tests for it!