Leading zero omitted from zone

[MattBlissett]

This is a minor issue, thanks for providing this library.

I'm not sure if the NGA site (under Grids/Graticules → MGRS) is the specification for MGRS, but it says:

The first two characters represent the 6° wide UTM zone:

• Leading zeros are included so that Zone 9 is "09".

This library does not include the leading zero, zone is an integer here:

mgrs-java/src/main/java/mil/nga/mgrs/MGRS.java

Lines 243 to 250 in 5eb9daf

	public String coordinate(GridType type) {
	StringBuilder mgrs = new StringBuilder();
	if (type != null) {
	mgrs.append(zone);
	mgrs.append(band);