RegExp: improved source string treatment.

RegExp: improved source string treatment.

Previously, njs_regexp_pattern_create() in addition to a pattern
compilation made a string representation for a RegExp which was returned
by RegExp.prototype.toString() as is.

After 02444445df29 (0.6.0), RegExp.prototype.toString() was implemented
according to the spec, and since then it creates a RegExp string on the fly.

This patch removes the extra code which was left.

In addition, as a source string may not be a valid UTF-8 string (in
RegExp literals), RegExp.prototype.toString() now ensures that a
valid UTF-8 string is returned.