force contentSizeFlag=0 when using ZSTD_initCStream_usingCDict()

because by definition srcSize is not known when using this prototype.
added relevant test

Note : this use was already working, because at a later stage
(both ZSTD_compressBegin_usingCDict() and ZSTD_copyCCtx())
pledgedSrcSize=0 is translated into "unknown", no matter the frame parameter.
This is not correct, but of little importance,
as the medium term plan is to no longer set fParams within CDict

lib/compress/zstd_compress.c

@@ -3077,7 +3077,8 @@ static size_t ZSTD_initCStream_stage2(ZSTD_CStream* zcs,
 size_t ZSTD_initCStream_usingCDict(ZSTD_CStream* zcs, const ZSTD_CDict* cdict)
 {
     if (!cdict) return ERROR(GENERIC);   /* cannot handle NULL cdict (does not know what to do) */
-    {   ZSTD_parameters const params = ZSTD_getParamsFromCDict(cdict);
+    {   ZSTD_parameters params = ZSTD_getParamsFromCDict(cdict);
+        params.fParams.contentSizeFlag = 0;
         zcs->cdict = cdict;
         return ZSTD_initCStream_stage2(zcs, params, 0);
     }