Update RestTemplateBuilder.defaultHeader to use an array for a default header value instead of a single value

[nosan]

Update RestTemplateBuilder.defaultHeader to use an array for a default header value instead of a single value.
Prior to this commit, only a single value could be used.

It can be useful in case of when folks would like to specify a default header for Accept header. e.g.

		RestTemplate template = this.builder
				.defaultHeader(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE, MediaType.APPLICATION_XML_VALUE)
				.build();