Fix Kit rendering context error when render? is false

[stephannv]

Fix #979

I've just moved the ensure to wrap the code where Thread.current is modified.
eg.

# BEFORE
def internal_call(...)
  ...
  code
ensure
  ...
end

# AFTER
def internal_call(...)
  ...
  begin
    code
  ensure
    ...
  end
end

I could just move the previous_phlex_component = Thread.current[:__phlex_component__] to before the return "" unless render? too, but I think this code is clearer.