FrozenError - can't modify frozen String

[wjordan]

Version: haml master branch (no release version yet), tested on commit e4a8be9.

I produce the following stacktrace when precompiling haml template with tilt via a Sinatra application:

FrozenError - can't modify frozen String:
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:299:in `force_encoding'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:299:in `ensure in binary'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:299:in `binary'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:289:in `extract_magic_comment'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:285:in `extract_encoding'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:191:in `precompiled'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:251:in `compile_template_method'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:236:in `block in compiled_method'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:235:in `synchronize'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:235:in `compiled_method'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:169:in `evaluate'
	[ruby]/gems/tilt-2.0.8/lib/tilt/haml.rb:24:in `evaluate'
	[ruby]/gems/tilt-2.0.8/lib/tilt/template.rb:109:in `render'
	[ruby]/gems/sinatra-2.0.1.rc1/lib/sinatra/base.rb:839:in `render'
	[ruby]/gems/sinatra-2.0.1.rc1/lib/sinatra/base.rb:692:in `haml'

This issue seems to be caused by setting frozen_string_literal: true on lib/haml/temple_engine.rb (#967), which breaks an implicit contract in the interface between these two gems.

Whether this is a bug best fixed in tilt or haml (or both, for best backwards-forwards compatibility) I don't know, but in any case, tilt's Tilt::HamlTemplate#precompiled_template expects strings returned from calls to Haml::TempleEngine to be not-frozen, because it calls #force_encoding on the returned string.