Fix parsing text array with negative bounds

e.g. '[-4:-2]={1,2,3}'

fixes #132

array.go

@@ -305,7 +305,7 @@ func arrayParseInteger(buf *bytes.Buffer) (int32, error) {
 			return 0, err
 		}
 
-		if '0' <= r && r <= '9' {
+		if ('0' <= r && r <= '9') || r == '-' {
 			s.WriteRune(r)
 		} else {
 			buf.UnreadRune()

array_test.go

@@ -100,6 +100,14 @@ func TestParseUntypedTextArray(t *testing.T) {
 				},
 			},
 		},
+		{
+			source: "[-4:-2]={1,2,3}",
+			result: pgtype.UntypedTextArray{
+				Elements:   []string{"1", "2", "3"},
+				Quoted:     []bool{false, false, false},
+				Dimensions: []pgtype.ArrayDimension{{Length: 3, LowerBound: -4}},
+			},
+		},
 	}
 
 	for i, tt := range tests {