GDScript: Fix missing conversion for default argument values

modules/gdscript/tests/scripts/runtime/errors/bad_conversion_for_default_parameter.gd

@@ -0,0 +1,8 @@
+var weakling = 'not float'
+func weak(x: float = weakling):
+	print(x)
+	print('typeof x is', typeof(x))
+
+func test():
+	print(typeof(weak()))
+	print('not ok')

modules/gdscript/tests/scripts/runtime/errors/bad_conversion_for_default_parameter.out

@@ -0,0 +1,8 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: weak()
+>> runtime/errors/bad_conversion_for_default_parameter.gd
+>> 2
+>> Trying to assign value of type 'String' to a variable of type 'float'.
+0
+not ok