http: fix crash when setting a header value to a non-string with a newline

From conversation [here](https://groups.google.com/forum/#!topic/nodejs/S5aV7V_PRqw), it seems storeHeader() happily converts any value to a string, unless the value, when converted to a string, has a newline in it, in which case is crashes. This should fix that to also convert to a string when stripping newlines.