fixed PY-5058 When unicode/raw string literals are line-split, the u/r/ur prefix is missing

python/src/com/jetbrains/python/editor/PythonEnterHandler.java

@@ -101,8 +101,9 @@ public class PythonEnterHandler extends EnterHandlerDelegateAdapter {
         return Result.Continue;
       }
       if ("\\".equals(doc.getText(TextRange.create(offset - 1, offset)))) return Result.Continue;
+      String pref = element.getText().substring(0, prefixLength);
       String quote = element.getText().substring(prefixLength, prefixLength + 1);
-      doc.insertString(offset, quote + " \\" + quote);
+      doc.insertString(offset, quote + " \\" + pref + quote);
       caretOffset.set(caretOffset.get() + 3);
       return Result.Continue;
     }

python/testSrc/com/jetbrains/python/PyEditingTest.java

@@ -102,6 +102,11 @@ public class PyEditingTest extends PyTestCase {
     doTestEnter("# foo <caret>bar", "# foo \n# bar");
   }
 
+  public void testEnterInPrefixString() {  // PY-5058
+    doTestEnter("s = r'some <caret>string'", "s = r'some ' \\\n" +
+                                            "    r'string'");
+  }
+
   public void testEnterInStatement() {
     doTestEnter("if a <caret>and b: pass", "if a \\\nand b: pass");
   }