Change GroupWithGenerators to accept collections again

In GAP <= 4.9, this worked:

    gap> GroupWithGenerators( Group( (1,2) ) );
    Group([ (), (1,2) ])

In GAP 4.10, this was broken and instead lead to a "method not found" error.
While strictly speaking this was never documented behavior, we restore it
to avoid regressions in code that relied on this undocumented behavior.

Resolves gap-system#2703

lib/grp.gi

@@ -4401,13 +4401,14 @@ InstallMethod( GroupWithGenerators,
 function( gens )
 local G,typ;
 
+ gens:=AsList(gens);
  typ:=MakeGroupyType(FamilyObj(gens),
  IsGroup and IsAttributeStoringRep 
  and HasIsEmpty and HasGeneratorsOfMagmaWithInverses,
  gens,false,true);
 
  G:=rec();
- ObjectifyWithAttributes(G,typ,GeneratorsOfMagmaWithInverses,AsList(gens));
+ ObjectifyWithAttributes(G,typ,GeneratorsOfMagmaWithInverses,gens);
 
  return G;
 end );
@@ -4418,13 +4419,14 @@ InstallMethod( GroupWithGenerators,
 function( gens, id )
 local G,typ;
 
+ gens:=AsList(gens);
  typ:=MakeGroupyType(FamilyObj(gens),
  IsGroup and IsAttributeStoringRep 
  and HasIsEmpty and HasGeneratorsOfMagmaWithInverses and HasOne,
  gens,id,true);
 
  G:=rec();
- ObjectifyWithAttributes(G,typ,GeneratorsOfMagmaWithInverses,AsList(gens),
+ ObjectifyWithAttributes(G,typ,GeneratorsOfMagmaWithInverses,gens,
  One,id);
 
  return G;

tst/testbugfix/2018-12-06-GroupWithGenerators.tst

@@ -0,0 +1,5 @@
+# These are undocumented, but should still work
+gap> GroupByGenerators( Group( (1,2) ) );
+Group([ (), (1,2) ])
+gap> GroupWithGenerators( Group( (1,2) ) );
+Group([ (), (1,2) ])