Ignore when first constructor parameter is dropped from Signature

Before Scala 2.12.9/2.13.1, the `Signature` attribute of an inner class
constructor would also contain the first, synthetic parameter.

This was fixed in scala/scala#7975, which course leads
to mima warnings about inconsistent signatures. Since the descriptor remains
the same, and the previous signature was wrong anyway (so anything relying on
it can not have worked in the first place) this specific change can be safely
ignored

Tested with Akka HTTP, akka/akka-http#2655 (comment)

core/src/main/scala/com/typesafe/tools/mima/lib/analyze/method/MethodChecker.scala

@@ -35,9 +35,10 @@ private[analyze] abstract class BaseMethodChecker extends Checker[MethodInfo, Cl
   }
 
   private def methSigCheck(oldmeth: MethodInfo, newMeth: MethodInfo): Boolean = {
-    oldmeth.signature == newMeth.signature || (
-      newMeth.bytecodeName == MemberInfo.ConstructorName && newMeth.signature.isEmpty
-    )
+    oldmeth.signature == newMeth.signature ||
+      (newMeth.bytecodeName == MemberInfo.ConstructorName &&
+        (newMeth.signature.isEmpty || (oldmeth.signature.endsWith(newMeth.signature.drop(2))))
+      )
   }
 
   private def uniques(methods: List[MethodInfo]): List[MethodInfo] =