net.http: add a Header.starting_with method, to get the first header, starting with a key (#10119)

vlib/net/http/header.v

@@ -469,6 +469,16 @@ pub fn (h Header) get_custom(key string, flags ...HeaderQueryConfig) ?string {
 	return h.data[data_key][0]
 }
 
+// Gets the first value of the header starting with key, or none if the key does not exist.
+pub fn (h Header) starting_with(key string) ?string {
+	for k, _ in h.data {
+		if k.starts_with(key) {
+			return k
+		}
+	}
+	return none
+}
+
 // Gets all values for the CommonHeader.
 pub fn (h Header) values(key CommonHeader) []string {
 	return h.custom_values(key.str())

vlib/net/http/header_test.v

@@ -115,6 +115,14 @@ fn test_get_custom() ? {
 	}
 }
 
+fn test_starting_with() ? {
+	mut h := http.new_header()
+	h.add_custom('Hello-1', 'world') ?
+	h.add_custom('Hello-21', 'world') ?
+	assert h.starting_with('Hello-') ? == 'Hello-1'
+	assert h.starting_with('Hello-2') ? == 'Hello-21'
+}
+
 fn test_custom_values() ? {
 	mut h := http.new_header()
 	h.add_custom('Hello', 'world') ?