fix:set_file_content with range request return 416.

[sinnren]

in function "detail::range_error",check range pos and content_length:
// Range must be within content length if (!(0 <= first_pos && first_pos <= last_pos && last_pos <= contant_len - 1)) { return true; }

use res.set_file_content in callback, set "content_length_" in res.set_content_provider function, but res.set_content_provider is after "detail::range_error", cause "detail::range_error" return true(error).

to solve this problem，put "detail::range_error()" after file content served, it's ok.

[yhirose]

@sinnren thanks for your contribution!