Ruby: String.try_convert isn't value-preserving

`String.try_convert` can convert arbitrary objects to strings, which
obviously isn't value-preserving.

ruby/ql/lib/codeql/ruby/frameworks/core/String.qll

@@ -47,7 +47,7 @@ module String {
     override predicate propagatesFlowExt(string input, string output, boolean preservesValue) {
       input = "Argument[0]" and
       output = "ReturnValue" and
-      preservesValue = true
+      preservesValue = false
     }
   }
 

ruby/ql/test/library-tests/dataflow/string-flow/string_flow.rb

@@ -5,7 +5,9 @@ end
 
 def m_try_convert
     a = source "a"
-    sink String.try_convert(a) # $ hasValueFlow=a
+    b = source 1
+    sink String.try_convert(a) # $ hasTaintFlow=a
+    sink String.try_convert(b) # $ hasTaintFlow=1
 end
 
 def m_format