Make Unmarshal work with AutomaticEnv

[sagikazarmark]

• Fixes Consider env vars when unmarshalling #188
• Fixes Does viper require a config file in order to use ENV variables? #584
• Fixes Unmarshal using ENV without binding #688
• Fixes Commented out property in YAML yields inconsistencies between Get and Unmarshal when overriding using environment variables #725
• Fixes Unmarshal non-bound environment variables #761
• Fixes UnmarshalKey / Sub only uses read config, neglects environment variables  #1012
• Fixes Viper does not unmarshal values passed from env or pflag #1365
• Fixes AutomaticEnv function failing to read environment variables in Go code #1522
• Fixes Use environment variables with no need to have config file (yml,josn,etc...) #1587

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• Fixes Implement viper.BindStruct for automatic unmarshalling from environment variables #1429

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[sagikazarmark]

Fixed in #1429

[tomqwpl]

Whilst this appears to have fixed things for "Unmarshal", it doesn't appear to have fixed things for "Unmarshalkey". Naively I expect that Unmarshalkey to be consistent with Unmarshal. That is, the following test fails (added as part of TestUnmarshalWithAutomaticEnv:

        t.Run("Subkey", func(t *testing.T) {
            var config StorageConfig

		if err := v.UnmarshalKey("filesystem", &config); err != nil {
			t.Fatalf("unable to decode into struct, %v", err)
		}

		assert.Equal(
			t,
			StorageConfig{
				Size: 4096,
			},
			config,
		)
	})