Add unit test for the ExecutionContext.get methods

[Solodye]

These are the unit tests for #718

They can cover the added 2 ExecutionContext.get methods.

• spring-batch/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/ExecutionContext.java

  Lines 270 to 276 in 53e33c0

  	public <V> V get(String key, Class<V> type) {
  	Object value = this.map.get(key);
  	if (value == null) {
  	return null;
  	}
  	return get(key, type, null);
  	}

• spring-batch/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/ExecutionContext.java

  Lines 290 to 300 in 53e33c0

  	public <V> V get(String key, Class<V> type, @Nullable V defaultValue) {
  	Object value = this.map.get(key);
  	if (value == null) {
  	return defaultValue;
  	}
  	if (!type.isInstance(value)) {
  	throw new ClassCastException("Value for key=[" + key + "] is not of type: [" + type + "], it is [" + "("
  	+ value.getClass() + ")" + value + "]");
  	}
  	return type.cast(value);
  	}

[fmbenhassine]

LGTM 👍 Rebased and merged as a64b44e. Thank you for your contribution!