size_format doesn't work for TB

[nbachiyski]

size_format runs the number through int_val, which casts it to int, which is usually only 32 bits. Not enough for terabytes.

[tellyworth]

simpler patch with doubleval()

[tellyworth]

Simpler patch included - this one just uses doubleval() in place of intval().

Both pass unit tests incidentally - see test_size_format() in ​http://svn.automattic.com/wordpress-tests/wp-testcase/test_includes_functions.php.

[darkdragon]

Can this go in?

[ryan]

(In [6669]) doubleval bytes. Props tellyworth. fixes #5246