Allow Ranges on Rash to match _all_ Numerics

Previously, it checked only for Integers; unfortunately,
that left Floats out, so passing in a float would cause
it to look it up normally.  This also changes the range
checking to `cover?` instead of `include?`.  Also
adds a line to a test to check for float availability.
Merge pull request #184 from medcat/rash-numeric-access

Allow Ranges on Rash to match all Numerics