Revert "Return XPath integers as actual integers."

The particular case of string(10) having to return "10" instead of "10.0" will
be handled separately. Returning integers breaks behaviour/expectations
elsewhere.

This reverts commit 431a253000cf2444785db0e12f1ce57e3742e312.
Proper handling of decimals for string() calls.

When calling the string() XPath function floats with zero decimals (10.0, 5.0,
etc) should result in a string without any decimals. Ruby converts 10.0 to
"10.0" whereas XPath expects "10".