Fetch: fixed heap-buffer-overflow in Headers.get().

Fetch: fixed heap-buffer-overflow in Headers.get().

Previously, when more than one header with the same name added to a
Headers object and Headers.get() was used to get the the duplicate
header heap-buffer-overflow occured. The overflow occurred due to an
incorrect calculation of the combined header value's length.

The issue was introduced in c43261bad627 (0.7.10).