country/code_fetcher.go: prevent records with smaller assignment priority from overwriting those with higher priority

[xgoffin]

What does this PR do?

We have two United Kingdoms:

                {
                        Name:        "United Kingdom",
                        Alpha2:      "GB",
                        Alpha3:      "GBR",
                        Numeric:     826,
                        DialingCode: "+44",
                        Assignment:  OfficiallyAssigned,
                },

		{
			Name:           "United Kingdom",
			Alpha2:         "UK",
			Alpha3:         "",
			Numeric:        -1,
			DialingCode:    "+44",
			Assignment:     ExceptionallyReserved,
			AlternateNames: []string{"Northern Ireland"},
		},

Latter overloads the former in Name

Fixes #

What are the observable changes?

Good PR checklist

• Title makes sense
• Is against the correct branch
• Only addresses one issue
• Properly assigned
• Added/updated tests
• Added/updated documentation
• Properly labeled

Additional Notes