BoxingDeclarationVisitor #1120: simplify the boxing code for method specifiers now that Specification.declaration exists

Also don't forget to handle the boxing for specified method's return and attributes.
And don't throw for invalid input that should be detected by the typechecker.
IssuesTest #1037: don't expect backend bugs, just wait for the typechecker to fix ceylon/ceylon-spec#625
Test for #1120: Bug with type inference