chore: upgrade to ehttpc 0.4.7

[zmstone]

Fixes EMQX-8974

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• [~] Added tests for the changes
• [~] Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md and .zh.md files
• For internal contributor: there is a jira ticket to track this change
• [~] If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• [~] Schema changes are backward compatible

[coveralls]

Pull Request Test Coverage Report for Build 4356505127

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 38 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.03%) to 80.823%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_gateway/src/mqttsn/emqx_sn_frame.erl	1	63.48%
apps/emqx/src/emqx_flapping.erl	1	85.11%
apps/emqx/src/emqx_session.erl	1	87.6%
apps/emqx/src/emqx_stats.erl	1	91.67%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	2	68.06%
apps/emqx/src/emqx_cm.erl	2	91.28%
apps/emqx/src/emqx_channel.erl	4	87.27%
apps/emqx/src/emqx_sys_mon.erl	5	86.46%
apps/emqx/src/emqx_reason_codes.erl	21	83.09%

Totals
Change from base Build 4354409766:	-0.03%
Covered Lines:	24107
Relevant Lines:	29827

---

💛 - Coveralls

[kjellwinblad]

Here are steps to reproduce the original issue that is fixed by this PR:

1. Create a webhook bridge with:
   • header "content-type: text/html"
   • empty body
   • request type can be POST but I think one can reproduce it with the other request types as well
   • set all pool sizes (buffer worker pool and connection pool) to one to make sure all requests go to the same gun connection
   • Set up a rule that listen to a topic and connect it to the webhook bridge
2. Send two messages to the rule's topic
3. You should know see the error log message provided in the Jira issue in the log file