Silence a double->float precision loss warning.

This warning originated in Ia4c534ae27fe175dc7ba92f45a27a2306cc9ed49 with the change of one of the as_float instances's implementation from strtof->strtod (and likewise for the wchar_t versions).
Merge pull request #138 from awong-dev/fix_asfloat_warning

Silence a double->float precision loss warning.