Preserve header count when a value is replaced
Preserve header count when a value is replaced

Fix for #217, a regression added in #205. The "header_count"
field was incremented even when we overwrote a header entry, which
caused Webdis to send a header with no name and no value.

nicolasff/webdis,nicolasff/webdis,nicolasff/webdis,nicolasff/webdis,nicolasff/webdis