force contentSizeFlag=0 when using ZSTD_initCStream_usingCDict()

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