Fix FAKE Chinese text bug

See
fsprojects/FAKE#1196,
fsprojects/FAKE#1212,
fsprojects/FAKE#1213 &
fsprojects/FAKE#1194

Not sure of the ramifications of this change, but this fixes the
encoding issues in FAKE for me.

l.Head.Encoding was
`System.IO.StringWriter(new System.Text.StringBuilder()).Encoding`
which evaluates to utf-16
/cc:@forki

src/source/Yaaf.FSharp.Scripting/YaafFSharpScripting.fs

@@ -1180,7 +1180,7 @@ module internal Helper =
     override __.Dispose (r) =
       base.Dispose r
       if r then doAll (fun t -> t.Dispose())
-    override __.Encoding = l.Head.Encoding
+    override __.Encoding = Encoding.UTF8
     static member Create l = new CombineTextWriter (l) :> TextWriter
   type OutStreamHelper (saveGlobal, liveOutWriter : _ option, liveFsiWriter : _ option) =
     let globalFsiOut = new StringBuilder()