remove behavior of copyHeaders to copy all headers

Previously, when no keys were specified, copyHeaders would copy all
headers from src to dst.  I believe this is a remnant of some old code,
as we don't actually ever use that behavior today.

I'm removing this as it seems too likely to accidentally pass along
headers that shouldn't be.  Instead, let's always be explicit about
which headers to copy (which is what we currently do anyway).

imageproxy_test.go

@@ -62,30 +62,12 @@ func TestCopyHeader(t *testing.T) {
 		},
 
 		// copy headers
-		{
-			dst:  http.Header{},
-			src:  http.Header{"A": []string{"a"}},
-			keys: nil,
-			want: http.Header{"A": []string{"a"}},
-		},
-		{
-			dst:  http.Header{"A": []string{"a"}},
-			src:  http.Header{"B": []string{"b"}},
-			keys: nil,
-			want: http.Header{"A": []string{"a"}, "B": []string{"b"}},
-		},
 		{
 			dst:  http.Header{"A": []string{"a"}},
 			src:  http.Header{"B": []string{"b"}, "C": []string{"c"}},
 			keys: []string{"B"},
 			want: http.Header{"A": []string{"a"}, "B": []string{"b"}},
 		},
-		{
-			dst:  http.Header{"A": []string{"a1"}},
-			src:  http.Header{"A": []string{"a2"}},
-			keys: nil,
-			want: http.Header{"A": []string{"a1", "a2"}},
-		},
 	}
 
 	for _, tt := range tests {