Support UTF-8 characters in feature name again

This commit reverts 0d59859.
Also see:
- microsoft#2226
- microsoft#2478
- microsoft#2229

I reproduced the issue and as @kidotaka gave us a great survey in microsoft#2226,
I don't conclude that the cause is UTF-8, but "an empty string (character)".
Therefore, I revert "throw error when meet non ascii (microsoft#2229)" whose commit hash
is 0d59859, and add support feture names as UTF-8 again.

include/LightGBM/dataset.h

@@ -8,7 +8,6 @@
 #include <LightGBM/config.h>
 #include <LightGBM/feature_group.h>
 #include <LightGBM/meta.h>
-#include <LightGBM/utils/common.h>
 #include <LightGBM/utils/openmp_wrapper.h>
 #include <LightGBM/utils/random.h>
 #include <LightGBM/utils/text_reader.h>
@@ -633,10 +632,6 @@ class Dataset {
     // replace ' ' in feature_names with '_'
     bool spaceInFeatureName = false;
     for (auto& feature_name : feature_names_) {
-      // check ascii
-      if (!Common::CheckASCII(feature_name)) {
-        Log::Fatal("Do not support non-ASCII characters in feature name.");
-      }
       // check json
       if (!Common::CheckAllowedJSON(feature_name)) {
         Log::Fatal("Do not support special JSON characters in feature name.");

include/LightGBM/utils/common.h

@@ -921,15 +921,6 @@ static T SafeLog(T x) {
   }
 }
 
-inline bool CheckASCII(const std::string& s) {
-  for (auto c : s) {
-    if (static_cast<unsigned char>(c) > 127) {
-      return false;
-    }
-  }
-  return true;
-}
-
 inline bool CheckAllowedJSON(const std::string& s) {
   unsigned char char_code;
   for (auto c : s) {

src/io/config.cpp

@@ -20,9 +20,6 @@ void Config::KV2Map(std::unordered_map<std::string, std::string>* params, const
     if (tmp_strs.size() == 2) {
       value = Common::RemoveQuotationSymbol(Common::Trim(tmp_strs[1]));
     }
-    if (!Common::CheckASCII(key) || !Common::CheckASCII(value)) {
-      Log::Fatal("Do not support non-ASCII characters in config.");
-    }
     if (key.size() > 0) {
       auto value_search = params->find(key);
       if (value_search == params->end()) {  // not set

tests/c_api_test/test_.py

@@ -58,7 +58,7 @@ def c_array(ctype, values):
 
 
 def c_str(string):
-    return ctypes.c_char_p(string.encode('ascii'))
+    return ctypes.c_char_p(string.encode('utf8'))
 
 
 def load_from_file(filename, reference):