Update pattern key for split pretokenizer (#38)

* Split change key

* Fix test

Author: jackzhxng

Diff for: src/pre_tokenizer.cpp

@@ -78,7 +78,7 @@ PreTokenizerConfig& PreTokenizerConfig::parse_json(const json& json_config) {
   type = json_config.at("type");
   if (type == "Split") {
     try {
-      pattern = json_config.at("pattern");
+      pattern = json_config.at("pattern").at("Regex");
     } catch (json::out_of_range&) {
     }
   } else if (type == "Digits") {

Diff for: test/test_pre_tokenizer.cpp

@@ -221,7 +221,8 @@ TEST_F(PreTokenizerConfigTest, Split) {
           .parse_json(json{
               {"type", "Split"},
               {"pattern",
-               R"((?i:'s|'t|'re|'ve|'m|'ll|'d)|[^\r\n\p{L}\p{N}]?\p{L}+|\p{N}{1,3}| ?[^\s\p{L}\p{N}]+[\r\n]*|\s*[\r\n]+|\s+)"},
+               {{"Regex",
+                 R"((?i:'s|'t|'re|'ve|'m|'ll|'d)|[^\r\n\p{L}\p{N}]?\p{L}+|\p{N}{1,3}| ?[^\s\p{L}\p{N}]+[\r\n]*|\s*[\r\n]+|\s+)"}}},
           })
           .create();
   assert_split_match(*ptok, "Hello World", {"Hello", " World"});