respect text encoding specified in argument (#20999)

* respect text encoding specified in argument

* update

Author: xiangyan99

sdk/core/azure-core/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- respect text encoding specified in argument (thanks to @ryohji for the contribution)  #20796
+
 ### Other Changes
 
 ## 1.19.0 (2021-09-30)

sdk/core/azure-core/azure/core/pipeline/transport/_aiohttp.py

@@ -353,7 +353,9 @@ def text(self, encoding: Optional[str] = None) -> str:
         ctype = self.headers.get(aiohttp.hdrs.CONTENT_TYPE, "").lower()
         mimetype = aiohttp.helpers.parse_mimetype(ctype)
 
-        encoding = mimetype.parameters.get("charset")
+        if not encoding:
+            # extract encoding from mimetype, if caller does not specify
+            encoding = mimetype.parameters.get("charset")
         if encoding:
             try:
                 codecs.lookup(encoding)
@@ -372,7 +374,7 @@ def text(self, encoding: Optional[str] = None) -> str:
                 )
             else:
                 encoding = chardet.detect(body)["encoding"]
-        if not encoding:
+        if encoding == "utf-8" or encoding is None:
             encoding = "utf-8-sig"
 
         return body.decode(encoding)