sm2加密报错

使用type = "EC" 生成的公钥，在加密时报错，报错信息如下：
`pkey:asymmetric_routine EVP_PKEY_encrypt_init: code: -2 evp/pmeth_fn.c:141:error:0608B096:digital envelope routines: EVP_PKEY_encrypt_init: operation not supported for this keytype`

代码如下：
```lua
local pkey = require "resty.openssl.pkey"
local priv_key, err = pkey.new({
type = "EC"
})
local pub_pem = priv_key:to_PEM("public")
local pub_key, err = pkey.new(pub_pem)

local plaintext = "测试消息"
local ciphertext, err = pub_key:encrypt(plaintext)
if not ciphertext then
ngx.say(err)
end
```