res_pjsip_header_funcs: Make prefix argument optional.

The documentation for PJSIP_HEADERS claims that
prefix is optional, but in the code it is actually not.
However, there is no inherent reason for this, as users
may want to retrieve all header names, not just those
beginning with a certain prefix.

This makes the prefix optional for this function,
simply fetching all header names if not specified.
As a result, the documentation is now correct.

Resolves: #230

UserNote: The prefix argument to PJSIP_HEADERS is now
optional. If not specified, all header names will be
returned.

(cherry picked from commit 2179082eaf)

res/res_pjsip_header_funcs.c

@@ -453,7 +453,7 @@ static pjsip_hdr *find_header(struct hdr_list *list, const char *header_name,
 static int read_headers(void *obj)
 {
 	struct header_data *data = obj;
-	size_t len = strlen(data->header_name);
+	size_t len = !ast_strlen_zero(data->header_name) ? strlen(data->header_name) : 0;
 	pjsip_hdr *hdr = NULL;
 	char *pj_hdr_string;
 	int pj_hdr_string_len;
@@ -475,7 +475,7 @@ static int read_headers(void *obj)
 	list = datastore->data;
 	pj_hdr_string = ast_alloca(data->len);
 	AST_LIST_TRAVERSE(list, le, nextptr) {
-		if (pj_strnicmp2(&le->hdr->name, data->header_name, len) == 0) {
+		if (!len || pj_strnicmp2(&le->hdr->name, data->header_name, len) == 0) {
 			/* Found matched header, append to buf */
 			hdr = le->hdr;
 
@@ -518,8 +518,13 @@ static int read_headers(void *obj)
 	}
 
 	if (wlen == 0) {
-		ast_debug(1, "There was no header named %s.\n", data->header_name);
-		return -1;
+		if (!len) {
+			/* No headers at all on this channel */
+			return 0;
+		} else {
+			ast_debug(1, "There was no header beginning with %s.\n", data->header_name);
+			return -1;
+		}
 	} else {
 		data->buf[wlen-1] = '\0';
 	}
@@ -758,11 +763,6 @@ static int func_read_headers(struct ast_channel *chan, const char *function, cha
 		return -1;
 	}
 
-	if (ast_strlen_zero(args.header_pattern)) {
-		ast_log(AST_LOG_ERROR, "This function requires a pattern.\n");
-		return -1;
-	}
-
 	header_data.channel = channel;
 	header_data.header_name = args.header_pattern;
 	header_data.header_value = NULL;