This fixes jruby#3019, where in certain cases it's possible to end up with
RubyString instances backed by very large unique ByteLists where only
a tiny portion of the bytes in the ByteList are actually needed.

What was happening is `modify` was being called on RubyString
instances too eagerly, resulting in unnecessary duplication of their
underlying ByteList instances instead of sharing when possible.

The two changes here are in `RubyString#newFrozen` and
`RubyString#resize` and I believe are more correct based on the C
implementation of these methods.

For `RubyString#newFrozen`, which roughly corresponds to
`rb_str_new_frozen` in
C (https://github.com/ruby/ruby/blob/v2_2_2/string.c#L932), the logic
looks to me to return the shared string and I don't see it calling
`str_make_independent`, which is roughly the same thing as
`RubyString#modify`. So, I just removed the call to `modify`.

For `RubyString#resize`, which roughly corresponds to to
`rb_str_resize` in C, shared strings are only made independent if
their length
differ (https://github.com/ruby/ruby/blob/v2_2_2/string.c#L2155). Thus,
instead of unconditionally calling `modify` here, I moved it into the
conditions that are true when the length differs.