Feed retrieval incompatibility with 2.7.0-RC4

[zsgsp]

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Context

2.6.0 VS 2.7.0-RC4, feed retrieval

Summary

A previously uploaded legacy feed cannot be retrieved with 2.7.0-RC4. It is problematic on 2 levels:

1. Retrieving legacy feed without specifying the feed type.
2. Retrieving legacy feed succeeds with specifying legacy feed, but if a feed specifies a site that loads a feed the legacy parameter is not "inherited".

Expected behavior

Support legacy feeds by default, i.e. no need to change client code after upgrade to 2.7.0 to keep working.

Actual behavior

{
"code": 404,
"message": "address not found or incorrect"
}

Steps to reproduce

Scenario 1: legacy feed

1. Double check: using 2.6.0 retrieve http://localhost:1633/bzz/025a15d77837fc40016e832bf2fd149acec062026d54cd74d5cec5f1c278f2e0/ -- works
2. Set version to 2.7.0-RC4
3. Retrieve feed without specifying swarm-feed-legacy-resolve parameter
4. Result: 404 -- should give the actual data.
5. (if the parameter is specified to legacy, then it works, but it required change in client code)

Scenario 2: legacy feed linked from legacy feed

1. Double check: using 2.6.0 retrieve http://localhost:1633/bzz/025a15d77837fc40016e832bf2fd149acec062026d54cd74d5cec5f1c278f2e0/ -- works
2. Set version to 2.7.0-RC4
3. Retrieve feed with specifying swarm-feed-legacy-resolve parameter
4. Top level site loads appropriately.
5. Iframe gives 404 -- should give the actual data (from http://localhost:1633/bzz/3fdd83fc5a15d5d5b0bfc1f11e9eacdc8eb8f9ba47a71d6d10f20c99ab9705d6/).

Possible solution