Fix missing escaping of backslashes in writing string value (#132)

* Fix missing escaping of backslashes in writing string value
* added test of backslash escaping
Fixes: waku-org/nwaku#3572

Author: NagyZoltanPeter

json_serialization/writer.nim

@@ -459,6 +459,7 @@ template writeValueStringLike(w, value) =
       of '\f': addPrefixSlash 'f' # \x0c
       of '\r': addPrefixSlash 'r' # \x0d
       of '"' : addPrefixSlash '\"'
+      of '\\': addPrefixSlash '\\'
       of '\x00'..'\x07', '\x0b', '\x0e'..'\x1f':
         s.write "\\u00"
         s.write hexChars[(uint8(c) shr 4) and 0x0f]

tests/test_writer.nim

@@ -293,6 +293,7 @@ suite "Test writer":
 
   test "escapes":
     check Json.encode("\x12") == """"\u0012""""
+    check Json.encode("""a\b""") == """"a\\b""""
 
   test "Empty object":
     type NoFields = object