* mu-str.[ch]: add mu_str_ascii_xapian_escape_in_place, for escaping some

Xapian fields; also add some tests

src/mu-str.h

@@ -138,7 +138,7 @@ char* mu_str_summarize (const char* str,
  * 'Latin-1 Supplement' and 'Latin Extended-A'
  *
  * @param str a valid utf8 string or NULL
- * @param downcase if TRUE, convert the string to lowercase
+ * @param downcase if TRUE, convert the string to lowercase 
  * 
  * @return the normalize string, or NULL in case of error or str was NULL
  */
@@ -153,12 +153,29 @@ char* mu_str_normalize (const char *str, gboolean downcase);
  * 
  * @param str a valid utf8 string or NULL
  * @param downcase if TRUE, convert the string to lowercase
- * 
- * @return the normalize string, or NULL in case of error or str was NULL
+ *  
+ * @return the normalized string, or NULL in case of error or str was
+ * NULL
  */
 char* mu_str_normalize_in_place (char *str, gboolean downcase);
 
 
+/**
+ * escape the string for use with xapian matching. in practice, if the
+ * string contains an '@', replace '@', single-'.' with '_'. Also,
+ * replace ':' with '_', if it's not following a xapian-prefix (such
+ * as 'subject:', 't:' etc, as defined in mu-msg-fields.[ch]).
+ * changing is done in-place (by changing the argument string). in
+ * any, case, the string will be downcased.
+ *
+ * works for ascii strings, like e-mail addresses and message-id.
+ * 
+ * @param query a query string
+ * 
+ * @return the escaped string or NULL in case of error
+ */
+char* mu_str_ascii_xapian_escape_in_place (char *query);
+
 /**
  * 
  * parse strings like 1h, 3w, 2m to mean '1 hour before now', '3 weeks