Fix NumericTokenStream::setDoubleValue() to use correct encoding.

The code used a simple cast to int64_t, which truncated the value and
made searching with decimal precision impossible.  Moreover, the
encoding was different from what NumericRangeQuery expected and search
results were wrong in other situations as well.

Fix by using NumericUtils::doubleToSortableLong(), as both
NumericRangeQuery and the original Java code do.
Merge pull request #60 from vslavik/fix-indexing-of-doubles

Fix NumericTokenStream::setDoubleValue() to use correct encoding.