patch 8.1.1878: negative float before method not parsed correctly

Problem:    Negative float before method not parsed correctly.
Solution:   Apply "!" and "-" in front of expression before using ->.
diff --git a/src/testdir/test_method.vim b/src/testdir/test_method.vim
index ba13bb4..8c5f35b 100644
--- a/src/testdir/test_method.vim
+++ b/src/testdir/test_method.vim
@@ -115,6 +115,11 @@
   delfunc Concat
 endfunc
 
+func Test_method_float()
+  eval 1.234->string()->assert_equal('1.234')
+  eval -1.234->string()->assert_equal('-1.234')
+endfunc
+
 func Test_method_syntax()
   eval [1, 2, 3]  ->sort( )
   eval [1, 2, 3]