Fix isLead logic in EmacsMuleEncoding.

See https://github.com/jruby/jruby/pull/684 for details.
Remove unused imports.
Throw an exception for US-ASCII when getting bytes for codepoint > 0x7f.

US-ASCII is a 7-bit encoding so when writing bytes for codepoints > 0x7f,
an exception should be thrown.  This is required so that Integer.chr
correctly raises a RangeException for invalid values.