Not the greatest error message on bad dotted-key insertion

[epage]

This is a regression compared to toml-rs found by cargo's tests

For the following document

[http]
ssl-version = 'tlsv1.1'
ssl-version.min = 'tlsv1.2'
ssl-version.max = 'tlsv1.3'

---- config::config_get_ssl_version_both_forms_configured stdout ----                                                                              
thread 'config::config_get_ssl_version_both_forms_configured' panicked at '                                                                        
                                                                                                                                                   
error: output did not match:                                                                                                                       
1   1     could not load Cargo configuration                                                                                                       
2   2                                                                                                                                              
3   3     Caused by:                                                                                                                               
4   4       could not parse TOML configuration in `[..]/.cargo/config`                                                                             
5   5                                                                                                                                              
6   6     Caused by:                                                                                                                               
7   7       could not parse input as TOML                                                                                                          
8   8                                                                                                                                              
9   9     Caused by:                                                                                                                               
10       -  dotted key attempted to extend non-table type at line 2 column 15                                                                      
    10   +  TOML parse error at line 3, column 1                                                                                                   
    11   +  |                                                                                                                                      
    12   +3 | ssl-version.min = 'tlsv1.2'                                                                                                          
    13   +  | ^                                                                                                                                    
    14   +Duplicate key `ssl-version` in `[]` table                                                                                                
    15   +