RSRP-469040: Extract Members To Partial: Extract to existing class creates new partial class.

JetBrains ReSharper Ultimate 2018.1 EAP 4 D Build 12.0.20180322.125946-eap04d.

Test solution is attached.

1. Invoke Refactor This -> Extract Members To Partial on ExtractMembersToPartial class in ExtractMembersToPartial.cs file (first one).
2. Set "ExtractMembersToPartial" (second one) as "Select existing part". Check DoSomething(string):void checkbox.
3. Click Next.

Actual result:
New partial class in same file is created.

Expected result:
DoSomething method is added to ExtractMembersToPartial class (second one) on ExtractMembersToPartial.cs.