In ix bid adapter, default to assuming media type is banner

[dumbmatter]

Type of change

• Bugfix
• Feature
• New bidder adapter
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Does this change affect user-facing APIs or examples documented on http://prebid.org?
• Other

Description of change

As described in #2574, this code:

if (utils.deepAccess(validBidRequest, 'mediaTypes.banner') ||
        validBidRequest.mediaType === 'banner') {
        bannerImp = bidToBannerImp(validBidRequest);
        bannerImps.push(bannerImp);
}

assumes that mediaType or mediaTypes is set. According to http://prebid.org/dev-docs/publisher-api-reference.html specifying the media type is optional and it should default to assuming it's a banner.

This PR makes it assume the media type is banner if it's not set.

Other information

#2574