Prior to this commit, if a SpEL expression invoked a method or
registered function that declares a String varargs argument, there were
sometimes issues with converting the input arguments into the varargs
array argument. Specifically, if the expression supplied a single
String argument containing a comma for the varargs (such as "a,b"),
SpEL's ReflectionHelper.convertArguments() method incorrectly converted
that single String to an array via the ConversionService, which
indirectly converted that String using the StringToArrayConverter,
which converts a comma-delimited String to an array. Thus, "a,b"
effectively got converted to a two-dimensional array ["a", "b"] instead
of simply ["a,b"].
This commit fixes this bug by avoiding use of the TypeConverter and
ConversionService for single arguments supplied as varargs when the
single argument's type matches the varargs array component type.
Closes gh-27582
bc657eb4d5