fix: respect -pr http11 flag by disabling HTTP/2 fallback in retryablehttp

[sharkqwy]

Description

Fixes #2240

When the user specifies -pr http11, httpx correctly disables HTTP/2 on the primary transport by:

1. Setting GODEBUG=http2client=0
2. Setting transport.TLSNextProto to an empty map

However, retryablehttp-go's internal HTTPClient2 (used as a fallback when HTTP/1.x encounters malformed HTTP/2 responses in do.go:63-64) still has HTTP/2 enabled via http2.ConfigureTransport. This silently upgrades connections to HTTP/2 and defeats the -pr http11 flag.

Fix

After creating the retryablehttp client, when Protocol == "http11", override HTTPClient2 to use the same HTTP/1.1-only transport. This ensures no HTTP/2 fallback occurs at any level.

Changes

• common/httpx/httpx.go: Set HTTPClient2 to use the HTTP/1.1-only transport when -pr http11 is specified

Verification

Before (with -pr http11):

• retryablehttp detects HTTP/1.x error → falls back to HTTPClient2 (HTTP/2 enabled) → connection upgraded to HTTP/2

After (with -pr http11):

• retryablehttp detects HTTP/1.x error → falls back to HTTPClient2 (HTTP/2 disabled) → stays on HTTP/1.1

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