Allow whitespace in Range headers

While investigating basho/webmachine#186 it was noted that a small but
non-zero percentage of clients embed whitespace in Range headers. It
appears this is valid:

From http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

    byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )

From http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2

    #rule
    ...separated by one or more commas (",") and OPTIONAL linear
    white space (LWS)...
    ...null elements are allowed, but do not contribute to the count
    of elements present. That is, "(element), , (element) " is permitted,
    but counts as only two elements...