Preserve the case of username and password in URL.build

CHANGES/880.bugfix.rst

@@ -0,0 +1 @@
+Preserve the case of username and password in URL.build when extracted from host.

tests/test_url_build.py

@@ -257,3 +257,14 @@ def test_build_with_none_query_string():
 def test_build_with_none_fragment():
     with pytest.raises(TypeError):
         URL.build(scheme="http", host="example.com", fragment=None)
+
+
+def test_build_with_case_sensitive_user_and_password():
+    url = URL.build(scheme='httPS', host='usER:passWORD@hostNAME', path='/paTH')
+    assert url.user == "usER"
+    assert url.password == "passWORD"
+
+
+def test_build_with_case_sensitive_user():
+    url = URL.build(scheme='httPS', host='usER@hostNAME', path='/paTH')
+    assert url.user == "usER"

yarl/_url.py

@@ -261,6 +261,12 @@ def build(
         elif not user and not password and not host and not port:
             netloc = ""
         else:
+            if "@" in host and not user and not password:
+                user_pass, host = host.split("@", 1)
+                if ":" in user_pass:
+                    user, password = user_pass.split(":", 1)
+                else:
+                    user = user_pass
             netloc = cls._make_netloc(
                 user, password, host, port, encode=not encoded, encode_host=not encoded
             )