shdict "safe_set" may still remove data

When using safe_set on a shared dict, setting a key to a large value (that won't fit in the shdict), will result in the key being removed completely from the shdict.

My assumption had been that safe_set would never override items when running out of memory, but upon closer inspection of the documentation I'm not entirely sure whether this is a bug or intended behavior (perhaps safe_set only prevents other items from being removed). If this is the intended behavior of safe_set, then maybe the documentation should be updated to be more explicit about this specific situation? I'd be happy to submit a documentation pull request if this is the case.

Here's a quick resty example (using OpenResty 1.13.6.2) that shows how setting the foo key on a shdict to something that's too large results in the removal of the original foo value (I would expect that the second safe_set call would fail, but I had expected that the original 128 byte value for foo would be retained instead of replacing it with nil):

$ resty --shdict 'testing 16k' -e '
local random = require "resty.random"

local ok, err = ngx.shared.testing:safe_set("foo", random.bytes(128))
print("1. safe_set ok: ", ok)
print("1. safe_set err: ", err)
print("1. get type: ", type(ngx.shared.testing:get("foo")))

print("") 

local ok, err = ngx.shared.testing:safe_set("foo", random.bytes(1024 * 17))
print("2. safe_set ok: ", ok)
print("2. safe_set err: ", err)
print("2. get type: ", type(ngx.shared.testing:get("foo")))'


1. safe_set ok: true
1. safe_set err: 
1. get type: string

2. safe_set ok: false
2. safe_set err: no memory
2. get type: nil

Thanks!

Start by reproducing the documented ngx.shared.DICT:safe_set behavior with the resty command and the oversized value shown in the issue. Compare the observed result with the safe_set documentation and determine whether retaining the existing value is intended; done means the behavior is clarified through the appropriate implementation, test, or documentation change.